You should just get some half toe clips to put on your existing pedals. These will keep your feet from slipping off, without getting too complicated about it. You can get plastic half clips almost any bike shop.

3 miles is 3 miles. You have to start somewhere. The more you ride, the longer you can go.

Well, you start out riding and like it and explore and go further and work your way up.

When I was in high school, I rode my bike to school a lot. It was about a mile, but that seemed like a long ways. But now I don't think anything about riding 12 or 15 miles after work, and make weekend rides quite a bit longer. It's just a matter of getting used to it and having fun.

I've not run into the pedal issue you mention. The majority of the cyclists on this forum use the clipless pedals where the shoes clip to the pedals. I use platform/ rubber block pedals, probably pretty similar to what you use. But I've never had problems with feet slipping, even with cheap pedals from Walmart (which fell apart, but that's a different story). I can see that happening maybe if you were riding in leather-soled boots or something. Anyway, they do make some pedals for BMX use that have little spiky thingies on them that will keep your feet from slipping. Different shoes or maybe modifying your pedaling techniques might be better answers, but the spiky pedals are available. Two things to note: Most single-piece cranks are one size thread (1/2"), and modern multi-piece cranks are a different size thread (9/16", I think). If in doubt, take the bike or the old pedal in when you go buy the new one. Second thing is the left pedal is reverse threaded; if you don't realize that, you can strip the threads trying to get it out. The pedals are normally marked with an L and R for that reason.

On the butt issue, there's a lot of variation on what can go on. I bought a cheap mountain bike from Academy, and it had the most miserably uncomfortable seat ever made. It was like riding on a 2x4. I thought it was because it was a little narrow saddle, and replaced it with a cruiser saddle, which worked okay for me. But since then, I've ridden another bike or two with fairly narrow saddles that were a lot more comfortable, and realize the first was just a bad saddle. Unfortunately, people vary a lot, and there's not any one saddle that works best. Part of the answer is finding the right sadle in the first place. Part of the answer is having it adjusted right. Part of the answer is getting used to the saddle. Part of the answer is the padded bike shorts that most serious cyclists wear (although I don't). But they're not going to make an uncomfortable saddle comfortable; they may improve it. But if your butt hurts after three miles, something more drastic probably needs to change.